Key Role of Equilibrium HONO Concentration over Soil in Quantifying Soil-Atmosphere HONO Fluxes

Nitrous acid (HONO) is an important component of the nitrogen cycle in the atmosphere. Photolysis of HONO generates hydroxyl (OH) radicals and significantly influences the atmospheric oxidative capacity. Early laboratory work discovered that soil nitrite, produced via biological nitrification or denitrification, can be an important HONO source. However, the laboratory-determined chamber HONO fluxes can largely deviate from those in the real world for the same soil sample. Therefore, quantification of soil fluxes of HONO in the atmosphere remains challenging. [HONO]*, the equilibrium gas-phase concentration over the aqueous solution of nitrous acid in the soil, has been suggested as a key parameter for quantifying soil fluxes of HONO; but it has not yet been well-validated and quantified. In this project, we present a method to retrieve [HONO]* by conducting controlled dynamic chamber experiments with soil samples and validate the existence of [HONO]* over the soil. We show that [HONO]* is a soil characteristic, independent of HONO concentrations in the chamber. Therefore, it is reliable to use [HONO]* to quantify soil fluxes of HONO. [HONO]* performs as an indicator of the potential of soil to be a source or a sink for atmospheric HONO and helps to better quantify the role of HONO fluxes of soil in the HONO budget and its implications on the oxidizing capacity in the atmosphere.